Fiber based cements consisting of ceramic fibers, a ceramic filler and a silica sol as the binder are well known in the field of refractory materials and the related industries like furnace manufacturing for many years/decades. In the field of Diesel exhaust after treatment systems the material silicon carbide has been established since many years as the most robust material for making Diesel particle filters. It can withstand high temperatures during regeneration, especially during thermo shock regeneration with temperatures in the range of 1100-1200° C. Due to this high temperature stability it has a huge advantage in respect to Cordierite, which is less heat resistant and shows the tendency to melt at temperatures below 1000° C. in presence of ash. In contrast to Cordierite, the coefficient of thermal expansion is with a value of 4.6×10−6/K high and therefore filters made of silicon carbide have to be built up from single segments, bonded together by an elastic assembling layer. This is especially for these type of filter systems relevant, where the thermal mass or specific weight of the honey comb filter segments are low due to a high porosity level or thin cell walls. To give a rough number: if the specific weight of the porous honey comb filter is less than 750g/l, a filter with the diameter of 5.66″ has to be segmented minimum into 4 segments, if a thermo shock test at a soot load of more than 10 g/l should not lead to any damage in form of cracks. For the production of Diesel particle filters by assembling of several honey comb filter segments the Know-How from the fiber based cements used in refractory applications was used and modifications have been developed. The following patents and patent applications describe these type of prior art solutions:
EP 0 816 065 A1 describes an assembly of ceramic honey combs with an elastic material, which contains an organic and an inorganic binder, inorganic particles and inorganic ceramic fibers.
EP 1382445 A2 describes an assembling of ceramic honey combs by a paste like sealing member with subsequent drying and curing. Due to the examples the paste like material is made of a silicon carbide powder, a silica sol and ceramic fibers.
EP 1382442 A2 describes an assembling layer consistent of an inorganic powder or whisker on the basis of silicon carbide or nitrides and a colloidal sol.
EP 1 306 358 A describes an assembling layer consistent of an inorganic powder, inorganic fibers, an inorganic binder and an organic binder, in which the fibers and the particles of the powders are connected which each other three dimensional. EP 1 270 202 A describes an assembling layer which is similar to the other above mentioned solutions.
EP 1 142 619 A describes an assembly of porous ceramic honey comb segments. The main characteristic feature is that the only material for building up the ceramic honey combs, is alpha-silicon carbide. The used material for adhering the segments is again made of silicon carbide powder, ceramic fibers and a silica sol.
EP 1 479 881 A1 describes a filter system, which is build up from many single honey comb filters which are bonded together by an assembly layer, where the coefficient of thermal expansion of the filter segments αF and that of the assembling layer αL follows the following equation: 0.01 <|αL−αF|/αF<1.0. This shall also be the case for a coating layer for the lateral surface of such an assembled filter. As one can see in the description, in all examples and also in all comparative examples, the assembling layer is build up by a material comprising silicon carbide powders, ceramic fibers and a silica sol. By adding special additives like foaming agents or hollow ash or hollow clay particles, the structure of the assembling layer was changed in that way, that the above given equation for the coefficients of thermal expansion is fulfilled in the temperature range between 300° C. and 900° C.
EP 1 719 881 A1 describes a filter system like the previous EP 1 479 881 A1.In addition it specifies that the specific heat capacity per unit volume of the assembling layer is lower than that of the filter material. EP 1 726 796 A1 describes the corresponding case for the coating of the lateral surface.
To close each second channel on the inlet side of a porous honey comb filter and the corresponding complementary channels on the outlet side all solutions in the state of the art use the same material as for the production of the honey combs. The plugging (closing the open channels) is always done before the final sintering process.
Al solutions for glues and cements for assembling porous honey comb segments due to state of the art describe material compositions, which contain silicon carbide as the inorganic filler, ceramic fibers and a colloidal silica sol as the binder. The inorganic fibers, in most of the solutions alumina silicate fibers, have the function to create a defined tensile strength and to allow crack deflection if necessary. Crack deflection is useful, if due to a high thermal gradient during a soot regeneration phase a crack in one of the filter segments occur. It prevents the crack from moving through the assembling layer into the next segment and subsequently though the whole filter. Cracks throught the whole filter, so called ring—off cracks can be avoided. Due to this functionality the usage of ceramic fibers is proven successful.
A major problem of fiber cements with silica sol is their high shrinkage during the drying and curing/hardening. If during the assembling process of the filter, segments are assembled in more than one layer in one step, the subsequent shrinkage can cause voids and cracks in the cement layer during the drying. If high temperatures of 1000° C. or more are generated in the filter during an active regeneration, additional sintering effects occur in silica sol based cements which increase the formation of cracks. These voids and cracks can reduce the functionality of the filter and could lead in the application later on to a complete push out of single filter segments (telescoping effect). It is therefore important to find a cement, which shows no or no significant shrinkage during the drying and hardening process and which does not show any sintering effect at elevated temperatures.